Using oversized PSV

[MechBRR]

I'm sizing PSV's for fire case on two heat exchangers located near an upstream vessel for which a PSV exists (isolation valves are installed in connected piping). By inspection the surface area of the heat exchangers is about two thirds of the vessel for which the PSV exists. I've been asked if we could use relief valves in the heat exchangers identical to the vessel PSV to minimize spares and to eliminate having to perform the calculations. Is there a problem with using slightly oversized relief valves? Thanks for your input!

 

[JoeWong88]

MechBRR,
Fire case for vessel, there is liquid filled, 2/3-phase in vessel or gas filled vessel. Similarly, there is liquid filled gas filled for HX (if cooler or heater) but 2-phase in HX (if condenser or evaporator). Comparing surface area of vessel and HX can not justify same PSV to be used.

For example, vessel is having 2 phase (Vapor & Liquid). It will based on wetted surface area (majority this is governing case). However, the HX may be a heater on the vapor outlet which is completely filled with gas. Gas expansion will occur. The load will be much smaller (normally).

So, it is difficult to judge from the "surface area" only.




JoeWong
Chemical & Process Technology

 

[MechBRR]

JoeWong, thanks for your reply. I can clarify a little bit here. The vessel mentioned is a liquid vessel whose relief valve was sized based on wetted surface area. The two heat exchangers needing overpressure protection are not the same. One is a condenser upstream of the stated vessel. It is obviously 2 phase. The other is downstream of the vessel and acts only to cool liquid product before it is sent to the distribution system.

Correct me if I'm off base, but here is my line of thinking. The second unit seems like wetted area is the way to proceed, the first unit would see a smaller load as there would not be vapor generation from heat input.

To my previous last question. If it is determined the relieving capacities required are less than that of the vessel, would there be a problem using the oversized valves?

Again, thanks very much for the input.

 

[JoeWong88]

MechBRR,
One of the problem link to oversized PSV is chattering. Chatterng could lead to seat damage, leakage, etc.

PSV normally can take load from 25-100% of it design load (subject to PSV type - only manufacturer (not vendor) can advise). So again, without knowing the relief load (especially the condenser), it is hard to say if the PSV will experience chattering.

I feel that you have high chance to use same size PSV. Just need some figure to justify it.

JoeWong
Chemical & Process Technology

 

[insult2injury]

If there is sufficient margin between the operating and set pressure and the only relief scenario is a fire, then there is probably no issue with using an oversized valve. In the event of a fire where the valve opens, there may be some additional damage to the valve, but it would most likely need service after a fire anyway.

I2I

 

[skearse]

The only thing I would add would be the "defenseability" of using the existing valves. I don't know if your process is subject to PSM or any other codes (OSHA, ASME/NB, etc.) but in that case you will have to make sure that if any outside agency audits your relief protection you can sufficiently defend the relief device choices (this is a good practice, anyway, regardless of any other code/compliance requirements).

 

[djack77494]

Regardless of whether or not you decide it is acceptable to use an oversized PSV, please don't assume a valve size. Develop the possible relief scenarios and size a PSV before you conclude that any particular size will be adequate.

 

[MechBRR]

Thanks everyone for your valued input. We will go through the proper analysis and if the capacities allow, we will use identical relief valves.

Cheers!
MechBRR

 

[moltenmetal]

Either I skimmed too quickly or the OP didn't clarify where the relief valve was discharging to. The impact on any system and piping handling the relief valves' discharge is the most important consideration here. If the relief valves discharge directly to atmosphere, oversizing the valve is no big deal.